This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Nearly half of Earth's atmospheric oxygen is produced by marine phytoplankton, namely phototrophic algae and cyanobacteria. Cyanobacteria, sometimes misnamed blue-green algae, are the simplest of all photosynthetic organisms and are the descendents of the ancient proto-symbiotic organisms that evolved into chloroplasts;as such, they are an excellent model for studying photosynthesis. The most numerically abundant species of these microbes is the pico-cyanobacterium, Prochlorococcus marinus. In this work, we use electron cryotomography to examine multiple strains of Prochlorococcus (MED4, MIT-9313, and Natl-1A) and the bacteriophages that infect them. In addition, we also determine the structure of the isolated phages that infect the bacteria. We aim at obtaining a snap shots of high resolution structures of the phages during the infection process. This will require a combination of electron cryotomography and single particle averaging.